It is known to print labels for containers by reverse-printing unchanging indicia, such as brand names, on the underside of a polyester layer adhered to a metal foil, and then printing by thermal transfer, variable product-related data, such as lot numbers, onto the external surface of the polyester layer. See, for example, the methods and laminates described in U.S. Pat. No. 5,653,844 and U.S. Pat. No. 5,656,362. Such labels are particularly useful for containers of contact lenses, where the labels appear on the exposed top surface of the lid.
Although such techniques have proven very advantageous, there remains a possible area for improvement. That is, such labels, when used on containers of contact lenses, have to undergo a sterilization process. Most preferable is a steam sterilization process as described, for example, in U.S. Pat. No. 5,488,815. In that process, lens containers are paired together (at B, FIG. 1) with the thermal print surface facing outward, then pushed into a slot in a storage box 100 where one of the print surfaces rests on the inside surface of the slot. The box with all its slots and lens containers in the slots is steam-sterilized in a steam chamber 150. But because this can cause transfer of the thermal print indicia from the lens container to the box 100, the box first must be flipped 90.degree. (step F of FIG. 1) so that the box slots are vertical and less contact occurs between the lens containers and the box slot surfaces containing them. Thereafter, the paired lens containers are removed from box 100 and packaged into a package 22 where they are stacked, each with the thermal print surface contacting the thermal print surface of the next adjacent container (FIG. 3).
One problem with such an approach is that even with the box slots (and contained lens containers) flipped to a vertical orientation, some contact can still occur between the thermal print surface and the slot surface during sterilization, leading to loss of label indicia. FDA regulations permit only a negligible occurrence of such failures. Furthermore, the process of the '815 patent goes to a lot of trouble to batch sterilize lens packages in a manner that minimizes, if not eliminates, contact of the thermal print surface of the lens package, with some other surface. Steam sterilization would be most efficient if practiced by steam-treating the package 22 with all of the plurality of stacked containers already in place, thus eliminating any intermediate storage box 100. Heretofore, steam-treating such a stack has not been possible, because of the transfer of thermally printed indicia to whatever the label contacts. (Hereinafter, "transfer surface" refers to a surface that is capable of having a thermally printed label transfer to it under heated conditions.)
Thus, there has been a need prior to this invention to provide a method which will allow steam-sterilization of contact lens containers while they are stacked, without destroying their labels.